Epiphenomenalism is a theory concerning the relation between the mental and physical realms, regarded as radically different in nature. The theory holds that only physical states have causal power, and that mental states are completely dependent on them. The mental realm, for epiphenomenalists, is nothing more than a series of conscious states which signify the occurrence of states of the nervous system, but which play no causal role. For example, my feeling sleepy does not cause my yawning — rather, both the feeling and the yawning are effects of an underlying neural state.